bug(test): four helpers resolve to nothing and report success — one imports nothing while claiming to import a module #15251

Found while fixing #15181. Four test helpers that resolve to nothing and report success, in three different shapes.

Why these are worse than a broken path

A wrong path that raises gets fixed the first time someone runs the test. These do not raise:

• _import_func masks its own defect — the anchor is off by one, but because the helper discards the result and returns None, nothing ever notices. Any assertion downstream is testing None. It is the shape test(quality): ten tests that could not fail — bare 'except Exception' swallowing AssertionError, and one asserting imports it never performed #15189 documented (a test that cannot fail), reached by a different route.
• A fixture returning a non-existent directory hands every consumer a path that silently yields no files. A test that iterates it passes by finding nothing — the exact defect bug(test): collection_coverage's file enumeration matches zero test_*.py files so 869 of them were never checked #15018 fixed one layer up, where an enumeration matched zero files and the guard reported clean.
• A sys.path entry pointing at nothing does not error; it just fails to make imports work, and whatever depended on it either skips or falls back.

Related, not duplicated

#15181 fixed one parents[N] anchor and added repo_tests/test_module_path_anchors_15181_test.py, an AST sweep over 111 anchored data-file expressions across the repository. It reaches modules by parsing rather than running, so it sees files hidden behind collection errors. It found exactly one unresolved anchor after the fix — the _import_func entry above, carried as its single grandfathered KNOWN_UNRESOLVED entry.

Directory-valued anchors are deliberately outside that guard, which is why the two test_data_dir fixtures are not caught by it. That exemption is worth revisiting as part of this issue: a fixture returning a missing directory is exactly as silent as a file anchor pointing at nothing.

Acceptance criteria

• _import_func either imports what it names or is removed. If anything depended on its None return, that dependency is the real finding — report it.
• Fix its parents[3] anchor, and remove the KNOWN_UNRESOLVED grandfather entry in the same change rather than leaving it recorded.
• Both test_data_dir fixtures point at a directory that exists, or are removed. Establish first whether anything consumes them — a fixture nobody uses is a different fix from one whose consumers have been quietly getting nothing.
• trivial_config_test.py:18 resolves to the real autobot-backend, and what it was supposed to enable is confirmed to work.
• Decide whether the anchor guard should cover directory-valued anchors. If not, record why here so the exemption is a decision rather than an omission.
• Contrast mutations for whatever guard covers these.

Refs #15181, #15018, #15189, #15178, PR #15249.